1. Field of the Invention
The present invention relates to a spiral bevel gear and a gear device used in an apparatus requiring high degrees of quietness and strength.
2. Description of the Related Art
A conical, straight bevel gear is known as a gear used to transmit motion between two intersecting axes.
Among bevel gears such as this, a bevel gear is disclosed in Patent Literature 1 that is used as a differential gear in an automobile and to which a high load is applied. In the bevel gear, a curvature radius of a connecting section (fillet curve) between a tooth flank and a bottomland that is conventionally constant based on a curvature radius of a small-diameter end section side is modified to gradually increase from the small-diameter end section towards a large-diameter end section. As a result, strength is increased in the large-diameter end section that had been prone to damage, allowing sufficient strength to be evenly ensured from the small-diameter end section to the large-diameter end section.
However, in Patent Literature 1, teeth knocking noise that occurs when the gear meshes with a partner gear is loud at the connecting section. Impact force applied to a tooth section during meshing is also large.
On the other hand, a technology is disclosed in Patent Literature 2 in which, in a similar differential gear, the connecting section between a tooth crest and a tooth flank of two straight bevel gears is rounded. In Patent Literature 2, as a result of a roundness M of a gear weaker than the partner gear being greater than that of the partner gear, a position at which the teeth come into contact is displaced to be closer to a tooth root. As a result, balance can be achieved between the strength of the two meshing gears. In Patent Literature 2, because the roundness M is actively provided, a contact area with the tooth flank of the partner gear can be increased. Quietness and strength during meshing can be enhanced.
In general, a spiral bevel gear is used instead of the straight bevel gear to improve quietness and strength.    Patent Literature 1: Japanese Patent Laid-open Publication No. 2002-295642    Patent Literature 2: Japanese Patent Laid-open Publication No. 2007-162746
However, in the above-described Patent Literature 2, the roundness M is constant from the small-diameter end section to the large-diameter end section. Therefore, a maximum value of the roundness M is limited by a tooth form at the small-diameter end section. As a result, further increase in quietness and strength during meshing is difficult to achieve through further increase in the roundness M.